RU2473925C1 - METHOD OF DETERMINING ABSORBED DOSE OF β-RADIATION IN SOLID-STATE THERMOLUMINESCENT DETECTOR - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention is a method of estimating the cumulative dose of ionising β-radiation using solid-state thermoluminescent detectors and can be used in personal dosimetry and in monitoring the radiation environment in different conditions. The method involves heating said detector from room temperature while simultaneously measuring thermoluminescence intensity and subsequently estimating the absorbed dose from parameters of the obtained thermoluminescence curve, wherein the solid-state thermoluminescent detector used is monocrystalline aluminium nitride AlN; the detector is heated to temperature of at least 400°C, and thermoluminescence intensity is measured only within the wavelength range from 340 to 380 nm.

EFFECT: wider range of linearity of the dose curve, high accuracy of estimating absorbed dose of β-radiation, wider field of use of the method, wider range of methods of determining absorbed dose of ionising β-radiation in a solid-state thermoluminescent detector.

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Description

The invention relates to radiation physics, and in particular to methods for estimating the accumulated dose of ionizing β-radiation using solid-state thermoluminescent detectors, and can be used in personal and clinical dosimetry, when monitoring the radiation situation in various conditions.

The registration and measurement of the absorbed dose of radiation of various types is carried out using solid-state thermoluminescent detectors made of various materials, such as lithium fluoride LiF; calcium fluoride CaF ₂ , alumina Al ₂ O ₃ , calcium sulfate CaSO ₄ : Dy; Studies are underway to create radiation-sensitive media based on substances of different classes, in particular wide-gap oxide materials (BeO, MgO, SiO ₂ ) [V.S. Kortov, I.I. Milman, S.V. Nikiforov, Solid-state dosimetry, Izvestia TPU, 2000, T.303, issue 2, pp. 35-45].

However, there is a need for the use of solid-state detectors with enhanced tissue equivalence and better suitability for use in personal dosimetry, including the evaluation of the absorbed dose of β radiation when working with radioactive materials and in space conditions.

Known material such as aluminum nitride AlN - direct-gap material with a large band gap [L.I. Berger, Semiconductor materials, CRC Press, 1997, pp.123-124]. It is a heat-resistant, acid-resistant material in polycrystalline form suitable for use in high-temperature semiconductor devices.

Due to the unique combination of physical and electrical characteristics: high thermal conductivity, good electrical insulation properties, moderate coefficient of thermal expansion at a relatively low cost, aluminum nitride is used as a structural ceramic material in the manufacture of cases and substrates of integrated circuits, powerful transistors, absorbers, and end loads, including in space technology [V.I. Kostenko, V.S. Seregin, L.A. Groshkova, A.I. Vasilevich, Modern Information and Design Office Technologies, 2003, http://www.iki.rssi.ru/seminar/tarusa200406/3-19.pdf].

A study of the reflection and excitation spectra of blue luminescence of AlN crystals in the energy range 3–40 eV showed the possibility of using crystalline aluminum nitride in optoelectronics as LEDs in the ultraviolet region of the spectrum [Michailin VV, Oranovskii VE, Pacesova S., Pastrnak J., Salamatov AS, Physica Status Solidi (b) 58 (1973) K51].

It has been reported [Radiation Measurements, Volume 33, Issue 5, October 2001, Pages 731-735] that a ceramic material in the form of aluminum nitride doped with yttrium oxide (AlN-Y ₂ O ₃ ), when exposed to ultraviolet light and to determine the absorbed dose by a thermoluminescent method, has linear dose dependence. The possibility of using ceramic AlN-Y ₂ O ₃ for dosimetry of ultraviolet radiation was discussed. However, no systematic studies of the possibility of measuring the absorbed dose of beta radiation were performed. Aluminum nitride with yttrium has a reduced tissue equivalence to radiation. The above results associated with the irradiation of doped yttrium oxide ceramic aluminum nitride with ultraviolet light cannot be applied to pure aluminum nitride and to materials irradiated with particle radiation, in particular β radiation.

A known method for determining the absorbed dose of β-radiation in a solid-state thermoluminescent detector based on an anion-defective single crystal of aluminum oxide (TLD-500K), including heating the specified detector with speeds of 0.25 ÷ 20 K / s in the temperature range from 303 K to 473 K ( 30 ÷ 200 ° C) with simultaneous measurement of the intensity of the thermoluminescent glow during the heating process in a wide spectral region (from 300 to 800 nm, the entire visible spectrum) and the subsequent assessment of the absorbed dose by the parameters of the obtained thermal emission curve: either by the value of of the totals, or according to the peak intensity of the specified curve [I.I. Milman, S.V. Nikiforov, BCKort, A.K. Kilmetov, Quality control of radiation detectors for radiation defectoscopy, Flaw detection, 1996, No. 112, pp. 64-70 ].

The disadvantage of this method is the presence of non-linearity (superlinearity) of the dose dependence for absorbed doses of more than 0.192 ÷ 0.32 Gy.

The prototype of the proposed method is a method for determining the absorbed dose of β-radiation in a thermoluminescent detector based on an anion-defective single crystal of alumina, including heating the specified detector from room temperature to 300 ° C with simultaneous measurement of the intensity of the thermoluminescent glow in the visible spectrum during heating the range of wavelengths from 500 to 570 nm, and the subsequent assessment of the absorbed dose according to the parameters of the obtained thermal emission curve [RF patent 2378665].

The prototype method provides linear dose dependence with absorbed doses up to 1 Gy. The disadvantages of the method are the limited linearity of the dose dependence, the decrease in the accuracy of determination (estimation) of the absorbed dose at values exceeding 1 Gy, and the dependence of the parameters of the thermoluminescent peaks on the dose and the conditions for preliminary photothermal treatment of the material [IIMilman, VSKortov, SVNikiforov, Radiation Measurements 1998, Vol.29, No. 3-4, pp. 401-410]. The scope of the method is limited to higher absorbed doses.

The objective of the invention is to expand the range of linearity of the dose dependence and the corresponding increase in the accuracy of estimating the absorbed dose of β-radiation, reducing the dependence of the parameters of thermoluminescent peaks on the dose and conditions of the pre-photothermal treatment of the material, expanding the scope of the method, expanding the arsenal of methods for determining the absorbed dose of ionizing β-radiation in solid state thermoluminescent detector.

To solve this problem, the method for determining the absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector, which includes heating the specified detector from room temperature with the simultaneous measurement of the intensity of the thermoluminescent glow during heating and subsequent assessment of the absorbed dose from the parameters of the obtained thermal emission curve, differs in that as solid-state thermoluminescent detector used monocrystalline aluminum nitride AlN, heating detector lead to a temperature of at least 400 ° C, and the measurement of the intensity of the thermoluminescent glow is carried out only within the wavelength range from 340 to 380 nm.

The technical result of the invention is an increase in the upper value of the linear range of the dose dependence up to 5 Gy (Fig. 1) when measuring the absorbed dose of ionizing β-radiation. This is ensured by the implementation of a set of distinctive and restrictive features of the method, in particular, the use of a single crystal of aluminum nitride as a solid-state detector, heating the detector in the temperature range from 20 ° C to 400 and more than ° C, and measuring the intensity of the thermoluminescent glow only within the wavelength range of 340 ÷ 380 nm. EFFECT: increase of the upper boundary of the linear range of the dose dependence from 1 Gy to 5 Gy, that is, 5 times in comparison with the prototype. The dependence of the position of the maxima of thermoluminescent peaks on the dose is reduced. The conditions for preliminary photothermal processing of the detector material have a weak effect on its sensitivity.

The proposed method expands the arsenal of previously known methods for measuring the absorbed dose of ionizing β-radiation, improves the accuracy of estimating the absorbed dose in the dose range from 10 ^-5 to 5 Gy and expands the scope of the method in the direction of measuring increased values of the absorbed dose.

When measuring the intensity of the thermoluminescent glow at wavelengths less than 340 nm, the upper value of the linear range of the dose dependence significantly decreases. With an increase in the wavelength of more than 380 nm, the components of the thermoluminescent glow of the detector are recorded, introducing errors in the estimate of the absorbed dose and significantly reducing the accuracy of such an estimate. These components are caused by uncontrolled impurities of the material of the thermoluminescent detector, the thermal background, and the influence of deeply located traps of a single crystal of aluminum nitride.

The described relationship between the distinguishing features of the proposed invention and the new technical result is experimentally identified by the inventors.

The invention is illustrated by drawings:

figure 1 - obtained by the authors of the dose dependence with linearity that occurs with absorbed doses up to 5 Gy; this figure shows the dependences of the radiation intensity and light sum on the absorbed dose obtained for single-crystal aluminum nitride by the method of thermal stimulation; the horizontal axis represents the absorbed dose (Gy); values of radiation intensity in relative units (rel. units) are plotted on the left vertical axis, light sums (rel. units) on the right vertical axis;

figure 2 is a block diagram of a device for determining the absorbed dose of β-radiation in a thermoluminescent detector based on a single crystal of aluminum nitride;

figure 3 - obtained by the authors of the dependence of the intensity of thermoluminescence on the heating temperature at two values of the absorbed dose (1.92 and 3.84 Gy); the horizontal axis represents the temperature values (° C), the vertical axis represents the values of the intensity of thermoluminescence in relative units (rel. units).

A device for determining the absorbed dose of β-radiation (figure 2) includes a solid-state thermoluminescent detector 1 based on a single crystal of aluminum nitride AlN, a heating unit 2 of the specified detector 1 and a unit 3 for recording the thermoluminescent glow of the same detector 1. The output 4 of the block 3 for recording the thermoluminescent glow is connected with an input of 5 block 6 absorbed dose assessment. Between the mentioned recording unit 3 and the thermoluminescent detector 1, on the propagation path of the glow 7 of this detector, there is a wavelength separation unit 8 of the detected thermoluminescent glow, indicated in FIG. 3 as a filter. Block 8 of the selection of wavelengths of the recorded thermoluminescent glow is made with characteristics that provide the function of selecting wavelengths only within the range from 340 to 380 nm (glow 9).

The heating unit 2 includes a heating table, on which the detector 1 is placed, and a heating power adjustment device (not shown in the drawing).

Detector 1 is a sample of single-crystal aluminum nitride having a wurtzite type of lattice, specific resistance 10 ¹¹ ÷ 10 ¹³ Ohm · cm, thermal conductivity 3.2 W / (cm · K), crystal cell parameters and thermal expansion coefficient close to gallium nitride, as well as a dislocation density of less than 10 ³ cm ^-2 .

The wavelength separation unit 8 of the recorded thermoluminescent glow 7 is an optical glass filter (for example, UFS-8 type) that performs the function of isolating (passing through itself) the wavelengths of the thermoluminescent glow in the range 340–380 nm (glow 9). As block 8, an appropriate interference filter can be used.

Block 3 registration thermoluminescent glow 9 is a photomultiplier tube, for example, type FEU-39A with an amplifier and signal converter (not shown).

Unit 6 for estimating the absorbed dose is (not shown in the drawing) a microprocessor or personal computer (computer) with an interface for receiving a signal from unit 3 for registering the thermoluminescent glow of detector 1. Block 6 performs the functions of setting the temperature of detector 1, determining the values of the intensity of thermoluminescent glow 9 when preset temperature values, constructing a thermal emission curve (dependence of the intensity of the thermoluminescent glow 9 on the heating temperature of detector 1), determining cheniya lightsum said curve and evaluating the absorbed dose on the resulting value lightsum. The absorbed dose can also be estimated by the peak intensity of the thermal emission curve.

To control the heating of the detector 1, a control unit (not shown) is used, the inputs and outputs of which are connected to the power control device of the heating unit 2 and, through the appropriate interface, to the microprocessor or personal computer of the absorbed dose estimation unit 6. The function of said control unit may be performed by said microprocessor itself (personal computer).

In the computer of block 6, for measuring the absorbed dose, control programs for the measuring system, registration of thermal emission curves, and mathematical packages, in particular, Excel or Origin, are used.

The device works, and the method for determining the absorbed dose of β-radiation in a thermoluminescent detector based on single-crystal aluminum nitride is as follows.

The measured sample 1 (detector 1, figure 2) before starting the measurement has room temperature. The general technical concept of room temperature includes a temperature range of 20 ÷ 25 ° C, but a range of 17 ° C to 30 ° C can also be used. To determine the desired value of the absorbed dose of β-radiation, sample 1, if necessary, is heated to the first selected temperature value, for example, 25 ° C. The first temperature selected can be the room temperature that is valid in the room. Using a filter 8, a glow 9 in the ultraviolet range of 340 ÷ 380 nm is isolated from the thermoluminescent glow 7 of this sample 1. Using blocks 3 and 6, determine the intensity of the thermoluminescent glow at a set temperature and build the first point of the desired curve of thermal emission. Next, the detector 1 is linearly heated to the following temperature values and after set time periods (for example, 1 s), the subsequent points of the desired thermal emission curve are similarly constructed until a temperature limit of sample 1 of 400 ° C or more is reached. Heating is carried out at a speed selected in the range from 0.2 to 10 ° C / s. When the final heating temperature is less than 400 ° C, the accuracy of the estimate of the absorbed dose decreases. For example, at a final temperature value of 380 ° C, the corresponding part of the area under the curve of the dependence of the intensity of thermoluminescence on temperature, to the right of the temperature value of 340 ° C, is not taken into account (Fig. 3).

Data on the time elapsed since the beginning of the measurements, the temperature of the sample 1 and the intensity of its thermoluminescent glow 9, obtained using the described device, are recorded in the data file. The data file is processed by a mathematical package, for example, of the type Excel or Origin. From the desired heat-emission curve obtained, the desired light sum value or the desired peak intensity value of the specified curve is determined, from which the value of the desired dose of β radiation absorbed by sample 1 is estimated. For this, the measured sample 1, prepared for subsequent use (freed from the previously received absorbed dose of β-radiation), is exposed to a known reference value of the dose of β-radiation (of the order of 0.01 ÷ 0.05 Gy). Then, in the manner described above, the value of the reference light sum or the reference intensity of the peak of the thermal emission curve is determined. The desired value of the absorbed dose of β-radiation of sample 1 is calculated using block 6 estimates of the absorbed dose according to the following formulas:

;

;

or

,

,

Where

D _suit - the desired value of the absorbed dose of β-radiation, Gy;

D _reference - the reference value of the absorbed dose of β-radiation, set within 0.01 ÷ 0.05 Gy;

S _claim - the desired value of the desired curve lightsum thermoluminescence, rel. units;

S _reference - the reference value of the light sum of the reference curve of thermal emission, rel. units;

I _claim - the desired value of the peak intensity of the thermoluminescence of the desired curve, rel. units;

I _reference - the reference value of the intensity of the peak of the reference curve of thermal emission, rel. units

The table shows the results of measurements and estimates of the absorbed dose of β-radiation by the method using a single crystal of aluminum nitride AlN as detector 1 at three values of the test absorbed dose (0.0016 Gy, 0.48 Gy and 3.84 Gy). The reference value of the absorbed dose of β-radiation was taken equal to 0.03 Gy. The test and reference values of the absorbed dose in these samples were established by irradiating these samples at room temperature with β radiation of a ⁹⁰ Sr / ⁹⁰ Y source with a dose rate of 0.032 Gy / min at the sample location. The heating rate of the samples was 2 ° C / s. As the results of the application of the methods, the values of the errors in estimating the desired absorbed dose in percent relative to the reference absorbed dose are given. The permissible relative error in estimating the required absorbed dose is 15% (GOST 8.035-82 “GSI. State primary standard and state calibration scheme for measuring absorbed dose and absorbed beta dose rate”).

Table Absorbed radiation dose, Gy Final heating temperature, ° C Relative estimation error,% by light sum in intensity total error 0.0016 420 2,5 0.2 2,5 0.4800 400 3.0 10.0 10,4 3.84 420 3.8 8.4 9.2

Below are described numbered according to the rows of the table (top to bottom) examples 1, 2, 3 of the implementation of the proposed method for determining the absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector based on aluminum nitride. The total estimation error in the table is defined as the root of the sum of the squares of the errors in light sum and intensity, also presented in the table.

Example 1

The absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector 1 based on monocrystalline aluminum nitride AlN is determined by heating the irradiated detector 1 at a rate of 2 ° C / s, starting from room temperature (25 ° C) and ending with a temperature of 420 ° C. The intensity of the thermoluminescent glow is measured only within the wavelength range from 340 to 380 nm, with the desired absorbed dose of the irradiated detector 1 equal to 0.0016 Gy. As a result, the total error in the estimate of the desired absorbed dose is 2.5%, that is, it is permissible (less than 15%).

Example 2

The absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector 1 based on single-crystal aluminum nitride AlN is determined in the same way as in example 1, with the exception of heating the detector 1 to a temperature of 400 ° C, with the desired absorbed dose of the irradiated detector 1 equal to 0.48 Gr. The total error in estimating the absorbed dose is 10.4%, which is acceptable.

Example 3

The absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector 1 based on monocrystalline aluminum nitride AlN is determined in the same way as in example 1, with the desired absorbed dose of irradiated detector 1 equal to 3.84 Gy. The total error in the estimate of the required absorbed dose is 9.2% and is acceptable.

Claims (1)

A method for determining the absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector, comprising heating the specified detector from room temperature while simultaneously measuring the intensity of the thermoluminescent glow and then evaluating the absorbed dose according to the parameters of the obtained thermal emission curve, characterized in that the solid-state thermoluminescent detector is used monocrystalline aluminum nitride AlN, the detector is heated to a temperature of at least 400 ° C, and the measurement of the intensity of the thermoluminescent glow is carried out only within the wavelength range from 340 to 380 nm.

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